This state variable filter (a filter which encompasses a Low Pass, High Pass and Band Pass output filtering the same input signal) was built after watching a video by @Lauri_diy
I did not completely copy his setup though, I changed it a bit. @Lauri_diy uses two CD4069 ICs. He implemented a separate output for each of the LP, BP and HP outputs. I combined the BP and HP outputs in one output using a toggle switch to either select the high passed or band passed signal. Combining these makes that the filter can be build using one CD4069. Furthermore I added a CV input to control the filter resonance.
As you can see he uses 4 of the 6 inverter stages of IC1 and 3 of the 6 inverter stages of IC2. Combining the BP and HP outputs makes it possible to use only one CD4069 IC, which in times of chip shortages may not be a bad idea. In reality I did not expect to be using both the HP and BP outputs at the same time so …
As you can see on the bottom right my implementation differs mainly concerning the BP/HP output. On the bottom left I added a CV input using a vactrol for the filter resonance. Finally although the schematic shows a 1N4148 input diode I used a Schottky diode (BAT85) which allows for lower input voltages to be processed.
For anyone who finds hand drawn schematics difficult to read I have this much more abstract schematic drawing made in easy eda, the lack of a nice template for the 4069 making this difficult to read as well, I’m afraid.
Because the CD4069 can not operate on negative voltages the input stage contains a diode to block negative voltages. As a side effect, this drastically changes the sound of the input signal as it is always rectified. This filter therefore is not a filter in the traditional sense that it only dampens a selected spectral range of a signal, it also clearly distorts the signal, and when fed a square wave or saw tooth it often does that in a way resembling the sound of a TB303 .
While using the filter I noticed that the Resonance CV input had only a small impact on the resonance. By rearranging the vactrol, this got much better.
So in stead of using this:
Because of the inherent distortion of the device, the diode which prevents negative input voltages from reaching the CD4069 distorts any bipolar signal (and low level unipolar signals), calling this thing a filter I feel is a bit of a misnomer. In a rack with bipolar input signals, it might be better described as a diode distortion followed by a SVF which sculpts the distorted signal.
So after consulting the dudettes (and dudes) from marketing I decided to rename the thing.
From now on State-Var-Filt-O-Matic will be known as Distort-O-Matic
If you do not want the distortion caused by the diode, you can change the input circuit so that it adds a bias voltage of half the supply voltage and connect the input signal to that via a capacitor. I’ve chosen 47uF for that.
Jos, you may not be aware of it but these logic inverters have an automatic self biasing property. Whilst your bias network will work it is not strictly necessary.
See below, this demonstrates the self biasing property, in this case the inverter is powered from 5 volts so self biases at around +2V5
Having had a look Ive actually struggled to find anything suitable online. This is a very well established trick and was commonly shown in electronics magazines in the 70’s and 80’s
Bassically the cmos inverter will automatically bias itself at 1/2 of the supply voltage. It is important that the AC signal is capacitively coupled otherwise the biasing will be disrupted
Provided you have an adequately big input resistor 10K+ to limit the current the gates internal protection diodes will protect it from greater than +ve rail or less than ground rail (negative voltages)
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